Remote control system



Feb. 10, 1948.

P-. K. ECKHARDT REMOTE CONTROL SYSTEM Filed Jan. 25, 1947 4 Sheets-Sheet 1 liyl.

T lazuli HIS INVENTOR ATTORNEY Feb. 10, 1948( P. K. ECKHARDT 1 REMOTE CONTROL SYSTEM Filed Jan. 23, 1947 4 Sheets-Sheet 2 WP- rd and V0.24.

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Station fill INVENTOR Paul A. I ha d. [219. 2. BY

HIS ATTORNEY Patented Feb. 1948 ice REMOTE CONTROL SYSTEM Paul K. Eckhardt, OHara Township,

County,

Allegheny Pa., assignor to The Union Switch &

Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application January 23, 1947, Serial No. 723,739 8 Claims. (01. 177-353) My invention relates to remote control systems, and more particularly, to the communication or line circuits for such systems, and has as its principal object the provision of remotely controlled means for enabling the adverse effects of line or apparatus faults to be more promptly overcome;

My invention is particularly adapted for use in connection with centralized trafiic control systems for railroads in which codes of impulses are transmitted over line wires extending from a train dispatchers oflice or other suitable point of control through a plurality of stations, for selectively controlling track switches and-signals at the stations, and for indicating the condition of the station apparatus and reporting train movements or the like at the control office.

As disclosed herein, my invention is applied to a centralized traffic control system of the coded carrier current type such as is shown in Letters Patent of the United States No. 2,350,668, issued June 6, 1944, to George W. Baughman and Norman F. Agnew, for Remote control systems. In the system of this patent, a line circuit which may extend the length of a railroad division is divided into sections by electrical filters at one or more'points to prevent the passage of direct current from section to section while permitting the passage .of voice or'carrier frequency currents to provide communication channels extending over two or more sections or between the end terminals of the line. Each section of the line', in the territory controlled by the centralized traffic control system, is equipped with a central battery line circuit which provides parallel connections to coding units at the stations of the system located in the section. The equipment for each section also includes a coding unit at a suitable control point such as a train dispatchers oflice, usually at one end of the line. Each oflice coding unit is adapted to transmit control codes for governing the track switches and signals at the stations in its section and to receive indication codes reflecting the condition of the controlled devices and for also indicating at the control office, the movements of trains through its section. Each controlled section which is remote from the office is provided with a remote line unit by which the codes of direct current pulses are supplied to or received from the line wires of its section. The remote line units and the oilice coding units for the remote sections are connected to carrier control units which. serve torepeat'the codes in both diI'QC-x tions over the intervening line wires as carrier current pulses of different frequencies, thereby providing independent channels for two-way 1 communication between the office and the stations in the remote sections.

A feature of the system of the patent is the provision of changeover relays'CH which enable each carrier control .unit to be quickly replaced by a reserve unit, in the event of a fault, the relays CH at the remote control stations being controlled by codes transmitted from theoflice by the centralized traffic control system.

One object of my invention is'the provision of means for suitably controlling the changeover relay at a station remote from theofiice, in the absence of available coding apparatus controlled over the intervening line wires.

A feature of the system as disclosed herein is that the utility of the changeover relays has been greatly extended by arranging them'so that they are capable of placing in service spare filters, remote line units or other apparatus, so as to enable a much wider variety of faults to be quickly corrected, and further, by the expedient of adjusting the spare carrier control units to operate with a higher transmitting energy and lower received energy than the prescribed energy levels to which those normally in service are adju's'ted, so as to enable abnormal attenuation of the carrier line currents to be compensated for by remote control.

It hasbeen found for example that in the event of a broken line wire, the continuity of the carrier current channels is maintained to some extent through the capacity to other wires on the same pole line so that the increased attenuation remains within limits which can be compensated for by the insertion of an electronic repeater or by the expedient above mentioned.

A feature of my invention is'the provision of means for inserting electronic repeaters in the line circuit, either manually by remote control, or automatically in response to a break in the physical line circuit, in response to the operation of the changeover relays or of other similar relays controllable independently of the changeover relays.

A specific feature of my invention is the provision of a direct current circuit of reversible polarity and of variable current strength which is superimposed upon the carrier current channels extending from the office to a remote station, by means of which the integrity of the physical line circuit is continuously detected and indicated at the oil'ice, and which provides two independent channels for the manual control by legend.

of changeover relays or the like, which relays also operate automatically to insert a repeater or other compensating means in the line, in response to the occurrence of a line fault.

Other objects, features and purposes of my invention will be pointed out as the description proceeds. V r A I shall now describe one form of apparatus embodying my invention and shall then point out the novel features thereof in claims.

Referring to the accompanying drawings, Figs. 1 to 4, inclusive, taken together, "illustrate the line circuit connections for a typical centralized trafilc control system embodying my 'inventibh.

An explanation of the inteinaT'ci-rcuits or -d= tailed mode of operation of the various apparatus units is not needed for an understanding of my invention, and since these are shown in detail in the Patent No. 2,350,668, or in other patents'referred to therein, they are shown here in block form to simplify the drawings, and areidentifi'e'd The linecircuit for the system com prises a pair of 'line 'wires Y- Z, which provide a through telephone "circuit comprising the secticns Yl' zl, Y2--Z2-, e'tc., connected by electribal filters-which serve to block theipass'a'ge of direct current from section to-section. Certain of the filters, designated are arranged to pass voi'ce frequencies only, whileothers, designated VP and HPFT are arrangedt'o pass vole-e frequencies and also the higher fi'eque'iicles use'cl 'for c'airi'er current transmission. 7

The centralizea tra'me control system cemprises three independent s-e"ts of apparatus identifi'ed'by the senses An tion with the station's adiaici'iflt'he three "line s'ectio'ns SH---'Z2, Y3--Z3 "and fi -Z4,- respectively, and neir'resee'etive control station in'Figs lz, B and were con'nectecl over the intervening line sectionYl Yfw'ith the corresponding coding unit's at*"the--office, Fig. ljby carrier control units cc, as shown, Where reseive appareit'u s units are "provided, the units normally in service are designated "No. I, and the reserve u'nits placed in service by the operation of the changeover frela cnare designated No. 2 and where similar on ts are'pr'ovl'ded at theofiie andstations, those at lihe'bffi'beareliielitlfi eii the prefix 'Of" The carrier current filters selectively "responsive "to single frequencies, are designated '74, hand 73 for the control code fr'e'quenci'es, and f4, f5 and f6 fbr the corresponding ihfiiclaltioh Etfle fiquefici'e's elnp lbyedby the "centralized "tra'nie centI'OT SyS tem.

Controlcodesforthe first 'e. c. section are gormally sup lied by the ofiie e eodiiig unit {A, Fig. '1 frwlii'ch corresponds ts that in Fig. 1 or Patent No. 2,350,668) fave heel; eeiitaet f a "changeover relay 'ICI I, t0 the CC -ilfll'i; N0. O'I'AfGYE'r a i'lbilitwl'iiilh for ElfIibHityiS shown ill-file dr'awill'g as a single lilie trhliila fied -by an arrow "to ndicate the direction or "transmission, and "the CC unit No. UTA repeat's't hese control codes as pulses of frequencies if] over the channe l including back contacts 7) and c of relay "Ion, line wires Y1 nd Z! and iro-nt contacts b and c of {a normally energized changeover relay ZCH, to'the'CC unitNo il A of Fig.*-2, whichcauses the rem'o'tefliiie unit No. lA connected thereto to repeat the carrier current-control codes direct cur-rent .pulses from the line -battery A, over frpnt'coritacts' d a, I11 andg of relay TCH through a low pass filter IPF to the line wires SEQ-and Z2, from which they are selectively received by the station coding units of the first section, which are connected through low pass filters LPF across the line wires Y2-Z2 as shown in Fig. 3. The control codes for the second and third sections, transmitted by the office coding units 03 and 0C, are repeated by the CC units Nos.

01113 and 010 as pulses 'freii'uene'y 112 or is over the line wires Yl and Zl of Fig. 1, back contacts I) and c of relays RTP and 4CHP of Fig.

2, through the filter VP and HIPF No. IA, over back contacts d and e of relays iCHP and RTP to li-frewii'es Y2 and Z2. frequency -f2 a? andear relat Normally the codes of received over back contacts b H by the CC unit IB, of Fig. 3, which causes the remote line unit connected thereto torepeat them as direct current impulses of current from battery 80B supplied over back contacts i1 and e of relay H, to the line wires Y3 and Z3 for the second section, through a filter and back contacts fend a of relay 'EC H, from 'whi'chthey areieceived'by the station cod-- ing master the cone sectionfshown in Figs. codes of n'eqiieney $3 in line wires Y2 z2 normally pass over -back contacts!) and 'c of relay SCH through the filter VP and HPF No. IB, and thence over back contacts far'ldy of relay to 'line wires Y3 and 2 3, from which they "af'e normally received over ba'ck con'tact's b and correlaysc n by the CC unit "No. "lq'l i'g. 4, which eauseethe associated remote lihe'uiiit to repeat these 'c'odes a's pulses "of direct "current from battery 89C to thelin'e wires'Y l and'Z l oi the thi'rlzlsectioll' to govern the stati'on coiling units of that section.

Indication codes "initiated by the station couing units of the several "sections-are normally transmitted as direct current "pulses to the "No. l remote line "unit for the corresponding section, causing the associated CC unit No. to repeat them as pulses 'frequeney 16,15 or jBQWl'i'iCh are-traii 'initteu-through the eorrespoiidi ng'l lesigiiatedjfilter over'thelin'e wiresexterminate-the offi'ce'where they "are received the appropriate drape "cofding unit, as will reaall' "be ap arent. V 'The changeov r relays 10H, 2CH and at the e r e, Fi l, are "controlled by the leyers i-P zen-aria 31 B, and indications as 'tbwhe'thi the normal or reserve "set" of apparatus controlled the ome changeoverfrla'ys is "in service is prev ea 53'! the "lightingofahiiitlicatibn lafrip' Tl or 2,'2| or'2'z, e'tc. I'hechahgebverielaiv's 56H an Hat thereincte centrelsta'tidns ft'r t'h'e se ond and third sections are controlled by "the office levers "JPB "and BPBfby cones transmitted to the last station'bffthe first or s'ecohd sectiion, respectively, "eafch dffwliicha's shown, is identified by the "co'd'ecall 246, and their condition is indicated at the fofilc'e byindica't ion codes transmitted frorh these stations 'to light the indica tioh lamp is"! ers'z, or lamp 6| or '62, all or which orresponds to the arrangement described in Patent No. 2,350,668.

The changeover relay Fig. '2, at the reniote primal station 'for the "first section, and a second relay RT at the same location, are independently controllable by direct current suppliedto theline wires Y1 and Z1 from a battery 21), Fig. -1 at the oflice over contacts of the levers llP B and IPB, respectively. Relay RT'is a bi-ased polar -relay which holds its contact a openon'ly when energized by current-of "normal polarity,

the current supplied thereto by operation of lever 'IPB. .Relay RT releases automatically due to its deenergization in the event of a fault such as a brokenlinewire, in the line section Yl-ZI, to interpose an electronic repeater, shown in Fig. 2, in the carrier current channels which normally extend to section Y2 Z2 through the filters f2, f3, f and f6, as shown. Since the repeater units act to pass and amplify the carrier frequencies, it is necessary to cut out the VP and HPF filter which would act as a shunt on the repeater. A band pass filter VPF is insorted with the repeater to carry the voice fre quencies.

Relay 4CH is a marginal relay of the retained neutral type which does not release in response to a reversal of line polarity by lever IPB, but which may be released manually by reducing the voltage supplied by battery 20, by the operation of the lever 4PB, without releasing relay RT. Relay 40H upon releasing picks up a repeating relay 4CHP. Relay 4CH also releases automatically in the event of a. broken line wire in the line section Yl-Zl, and when released, inserts the spare CC unit No. 2A into the carrier current channels for the first section, to compensate in part, at least, for the increased attenuation of the line currentsof frequencies fl and f4 due to such fault, the spare or No. 2 CC units being normally adjusted as required for that purpose as already explained.

Relays 50H and SCH are also arranged to be operated automatically in the event of a broken line wire in section Y2--Z2 or Y3-Z3, respectively, to interposethe No. 2 CC units which they control. Relay 5CH .thus is capable of providing automaticcompensation for increasedattenuation of the currents of frequencies 12 and f5 due to a broken line wire in section Y2-Z2, and relay BCH functions similarly with respect to frequencies f3 and {6, provided the fault is in section Y3-Z3. 1

To operate the relays 50H and GCH automatically in response to a line fault in the adjacent line section, a repeating relay RP, controlled by the line relay R of the last station coding unit in that section, is provided, together with a slow release station disconnect relay RPP controlled thereby, these relays corresponding to the similarlydesignated relays shown and described in Letters Patent of the United States No. 2,376,569, granted May 22, 1945, to George 'W. Baughman, for Remote control systems. Re-

lay RP is energized over normally closed contacts of relays R and RPP and repeats the code operation of relay R. Relay RPP has a release period of about eight seconds and does not release during the normal operation of the system but only when relay R is steadily released, and upon releasing, relay RPP opens the circuit for relay RP and also completes a circuit to energize relay SCI-I. When the fault is removed, and relay R is reenergized, relays RP and RPP may be reenergized by the transmission of a control code to pick up the station delivery relay D as explained in Patent No. 2,376,569.

It will be seen, therefore, that means are provided for compensating for one break in the carrier current circuits which extend over only one line section and for compensating for two breaks at different locations in those whichextend over two or three sections. If desired, com-- pensation for a third break in the line wires for the channels having three sections may be provided by. arranging Fig. 3 like Fig. 2, and include "6 a second electronic repeater in the circuits for currents of frequencies j3 and f6 in place of a filter. The relay 2RT shown in Fig. 3 provides means for controlling such a repeater, relays SCH and 2RT being independently controllable in accordance with the positions of levers SP3 and 8PB by .codes transmitted to the last station of the first section and received by the station storage unit having the code call 246 as shown, which unit also transmits indications of the condition of the positions of these relays to control the lamps 5|, 52 and 8|, 82.

The condition of the relays 40H and RT, of Fig. 2, is similarly indicated by another station storage unit of the first section, having the code call 351 and controlled over the carrier channel having the frequency f4, as shown.

As explained in Patent No. 2,350,668, hereinbefore referred to, the coding units correspond to those of Letters Patent of the United States No. 2,229,249, granted January 21, 1941, to Lloyd V. Lewis, for Remote control systems, except for the line circuit modifications by which they are adapted to parallel operation. The numbered wires leading to the storage units in Figs. 2 and 3 correspond to those of Fig. 7 of the Lewis patent, and the relays controlled by the ofiice storage unitsin Fig. 1 of the present application correspond to the similarlydesignated relays of this patent.

Considering now the mode of operation of the protection. system of my invention, and assuming first the occurrence of faulty code transmission due to a local apparatus failure or to damage to one of the line-connected filters by lightning. The operator will first energize the ofiice changeover relay for the faulty section, which may remove the fault; if not, he will thenactuate the remotely controlled changeover relay at the control station for the section, which'generally will result in locating and clearing the fault. The No. l carrier current receiving apparatus is normally adjusted to a fixed energy level, and in order to minimize interference with other carrier current channels which may employ the same or other wires on the same pole line, the

" normal transmission level is limited to about 18 decibels above a one milliwatt base. This value may be temporarily exceeded under emergency conditions, due to the No. 2 units being adjusted to transmit at a higher level and receive at a lower level, and in the case above, the maintainer can readjust the spare CC unit and restore the normal transmission values upon his arrival at the office or at the faulty location, following notification by the operator.

It is obvious that the removal of a fault in the first section allows the remotely controlled relays 40H and RT for that section to indicate their position, and enables the'operator to regain control of the relays SCH and 2RT to correct a fault in the second section, and to control relay BCH to correct a fault in the third section.

Assuming next the occurrence of increased line loss in all the carrier line circuits due to a sleet storm, for example, the third section being the first to fail. This fault may possibly be corrected by energizing the office changeover relays, but the preferred procedure is to release relay RT to insert the electronic repeater in the longer circuits, this having the advantage that the normal energy level is not materially exceeded at any point, the energy level being such as to allow normal operation of the units for the first section, but not the succeeding sections.

- adamant ln'ithe event or a ibrolcen line "wire in may of the. sections, the-changeover. relays zat'lthe remote ends or the sections uiresesctuated eutomatica'lly. -If the break i's zinisection xii-zigzalayss;R,T.;e;nd "30H 'eremeleased andasrellreeny explained, the increasedettenueition m themrier currents idea to the ibxreak ds 'mitometicelly comp ensated for, flue-Ito rrtnerspecieil sxdjustment ofxthelspare 2C0 or to the .insertionz 'of the repeater, and if this rcomp'ensaiti'on'xsuflices, (the system continues to opereite nproperly and "the fault is manifested only by ithe lightingf lamps '42 and .12. hurthercompensatlonlmaysbe "provided, if necessarygby the "usuaslioarocedure o'f energizing "the *officechangeover. relay whenever a faI11b OCCl11 S,-3&S "ellreadyexplained, while iii the compensation is excessive, theisystem may readily be adjusted by the maintainer: tozrestorc the zootmal transmission Bevel, T'a;s will readily be :understood. When (a :break i'o'ccmszzln'lth'e l-inels'cirizfor one Of itheremot'e sections, commxmicaticn is "maintained with :only these stations in that section "which :are zlocatedsion the nfiice side of break, so "that 7 by transmitting "control codes to each of'zthe'tstations. in the defective section-and observing "which of .'tl1ese rtransmit return indications; fthe. approximateiocation of 4a break in the line can ibe'rrea'dily ascertained.

In View :of the foregoing description it 'vlrilll be clear that the apparatus of my invention sprovides facilities bywhich ascentralized traffic contnol system coveringnn extensive ztermtorymmay be maintained in substantially continuous rserfvicelirrespeot-ive of the occurrenceiof a wiuevariety of zpossiblexfalfltsdnethe.ccmmunication'sline circuits or Fin ztherapparatus connected thereto.

-.-Although :.-I shaver'herein :andi described one lform ot-apparatus embodyingznry "inyention, anus-understood that variousrchanges and rmodifications :beimade rthercin iwithin the-'cscop'e offithezappended claims without idepazntmg firom ia ndtscope of any vinvention.

Having; thus described lmy iinwention, what it claim is;

1. .In a "remote control system, a twowire llne circuit ex'ter-lding from an office t'o 'o. remote station, a chamgreover"relay at said "station Mor mally energized oy clirect -:ourre-n t supplied over said-line circuit fromas'ource at the office, code receiving means :at the-office responsive to car-rier current energy v supplied over said line circuit, means ait the -s'tatlon ".Yor' s'upplylng to :said line circuit -can ier current energy :ofone value when the changeover relay is picked up an'dor a' higher value when the changeover relay is released, whereby the release-of said changeover'lrelay due to a sfault :in said :line circuit results in can increase in the learner cum-em t energysupplied to the line circuit to compensate in par-t .at least to the increased .LtIZSZIISTXIlSSiOIZl-HUS'S Ldue to the fault.

:2; aremote control Isystem, in two wireiline cireuit extendingrfrom :an office :to :a' remotezstation, a changeover'relaysat saidstati'onxnormally energized byniredt ircurren t supplied over said line scirouitffromia; source I951; lthebifice, :xso'de Ireceiving means alt'tthe-station responsive to carrier current energy-supplied over :seiid lin'e circuit, means atizthe lo'flloe for'lsupplyln'g "to esaid Illne'circuit carrier currentwener'gyof za value normelly sumcient it"o operate-said' receivmg means, and means ior rendering said receiving means responsive to carrier 'iourreut energyore lower value when said-changeover relay is re'leased,

8 Wherby tlie release or :said changeover :reiay'due to a. ffelult in said line =ciroui't incrcase's the transmission gain ln sall'd lrecetvlngrlm'ee ns t'o oom- ,pensaite in part atilea'st to. theflncreesed trans- -m?issihn ilbss due "to said -In e'remote control swtem; :a two wire zline circuit :extetiding from an niflcelio e remotezstation; a changeover relay-at saidzzstation mentally energized lby rdirect nurr'ent supplied over s'alid line circuitifrom a Isourbe zatztlte ofiice, mode meceivin'g :means atthe orficelresporrsive to carrier current energy supplied :oversaid dine two sets of similar epparatuszatithe station dror'asupplying carrier current energy to the line'nli cuit to which said :rec'eivhl'g tm'e'ans is responsive, one ofxzwhich setsxis arranged fto supply energy-hi e normal walue :when thechangeover relaylislpicked up, andthe other to'fsupplyaenergylof azhigher valuemhen Ihe'Ichangeoi/fer" relayeis r'e1oased.,-so that if said changeover'relay .isureleas'efi due to a fruit in line circuit the carrier current energy supplied vthereto lincreased to compensate i-in partfat least to the increased transmission Floss clue to the 'dault, and manually operable means at the fofiicezforicontrolllngfithe nurrerltlsu'pplied over saidllinte circuitl'to render said changeover relay-subject to manual control from'said: ofllce.

*4. in a remote rccmtrol system, Ea two Wire line circuit 'extendi'ngfirom an office tofia' remote :station, a changeover. relay "at said station normally energized by direct current 1 supplied over lsaid linezcircuit fromaa source at ltheofficmnode :tremsmitting means :a't the oificefrorl supplying oaded carrier current tosaid-Iinewtfires, twotsets of similar apparatus at the stationrorxrecewingssai'd coded "carrier currents, one -of whichiisxresponsive to the (carrier current normally'l'suppli'edfito the line circuit "and is "connected thereto-when the-changeover relay 'ispickedlupitthe other heir-1g connected to "the'lline :whenithe'fchangeover relay isxreleased and --adjustedimo respond at a. lower energy level to compensate? fol-lithe impairedftransmission duefto a--=line7fiarult;1nanually operable :m'eans at the DfiiHG FfOII rcon'trolling the direct current :supplied rtossai'dwllne'. wires Btu enable either of said s'ets =ofiapparatus to be iconnectedto Esa'iddine Wines by the: manual control of said changeover relay,

5. In a remote 'control zsystempa twoi -w'ire fline circuit comprising 1a :sectlomextending vtiroltn an office to "a first'rstation' and a second section extending from said first station to a second station, 'a. changeover zrelayr atisaid ilrs't's'station normally energized rby 'directr-enurrent :asupplied over the first section-cf rsaidia'l'irxe :oircuitifitrom :a source at the clfice; code recelvingvuneaus at the oifice responsive to la carrieri current energ-y supplied :over-said' sline ciz=cuitfaneans a't the "second station rfor.supplyingmarrierxurrent to the secand isectionliof said :l'me circuit, airepea'tcr' atsaid first-station forrsupplying increased carrier current: energyrto'mhe firsts section or said line circuit whemsupplied with carrier "current energy from the second sect-ion of said Slime circuit-and meansttor establishing connections 'to'tpass carrier current from the second lsect'ionto the 'first section when' the changeover-relay 1's energized and to interpose said repeater-when the changeover relay is released, "whereby the release of said changeover rla-y due to e fault lnsaidiirs't seotionresults in an 'increase in "the rarr'ier'cur rent energy supplied thereto ,to compensate in part at *least to the increased transm'issi'on loss due to ='the ta'ult. v

6. In a remote control system; a 'two"wlre'=line circuit comprising a first section extending from an ofiice to a first station and a second section extending from said first station to a second station, a changeover relay at said first station normally energized by direct current supplied over the first section of said line circuit from a source at the ofiice, code receiving means at the second station responsive to carrier current energy supplied over said line circuit, means at the office for supplying carrier current to the first section of said line circuit, a repeater at said first station for supplying increased carrier current energy to the second section of said line circuit when supplied with carrier current energy from the first section of said line circuit, and means for establishing connections to pass carrier current from the first section to the second section when the changeover relay is energized and to interpose said repeater when the changeover relay is released, whereby the release of said changeover relay due to a fault in said first section results in an increase in the carrier current energy supplied to the second section to compensate in part at least to the increased transmission loss due to the fault.

'7. In a remote control system, a, two wire line circuit comprising a first section extending from an office to a first station and a second section extending from said first station to a second station, a first and a second changeover relay at said first station normally energized by direct current supplied over the first section of said line circuit from a source at the oifice, the first changeover relay being responsive to current above a minimum value of one polarity only, and the secnd to current of a lower value of either polarity, manually operable means at the office for independently governing the polarity and value of said current to selectively control said relays, two sets of code receiving means at the ofiice selectively responsive to carrier current energy of a first and of a second frequency, respectively, supplied to said line circuit, two sets of apparatus at said first station for supplying carrier current of said first frequency to the line wires, one of which sets is arranged to supply energy of a normal value when one changeover relay is picked up and the other to supply energy of a higher value when said one changeover relay is released, a repeater at said first station for supplying increased carrier current energy to the first section of said line circuit when supplied with carrier current energy from the second section of said line circuit, means at said second station for supplying carrier current energy of said second frequency to the second section of said line circuit, and means controlled by the other changeover relay at said first station for passing carrier current from the second section to the first section when such relay is picked up and for interposing said repeater when such relay is released.

8. In a remote control system, a two wire line circuit comprising a first section extending from an ofllce to a first station and a second section extending from said first station to a second station, a first and a second changeover relay at said first station normally energized by direct current supplied over the first section of said line circuit from a source at the office, the first changeover relay being responsive to current above a minimum value of one polarity only, and the second only to current of a lower value of either polarity, manually operable means at the ofiice for independently governing the polarity and value of said current to selectively control said relays, two sets of code transmitting appara-- tus at the office for supplying coded carrier current of a first and of a second frequency, respectively, to the first section of said line circuit, two sets of receiving apparatus at said first sta tion selectively responsive to carrier currents of said first frequency, one of which is responsive to the current normally supplied to the first section of the line circuit and is connected thereto when one changeover relay is picked up, the other being connected to said first line circuit section when said one changeover relay is released, said other set being adjusted to respond at a lower energy level to compensate for the impaired transmission due to a line fault, a repeater at said first station for supplying carrier current of said second frequency and increased energy to the second section of said line circuit when supplied with carrier current of said second frequency from the first section, receiving apparatus at said second station responsive to carrier current of said second frequency in said second line circuit section, means controlled by the other changeover relay at said first station for passing carrier current of the second frequency from the first section to the second section when such relay is picked up and for interposing said repeater when such relay is released, whereby such changeover relay may be released manually to increase the carrier current energy supplied to the second section without increasing that supplied to the first section to compensate for a fault in said second section and both changeover relays release in the event of a fault in the first section to effect automatic compensation for such fault, with respect to carrier currents of both frequencies.

PAUL K. ECKHARDT. 

